Astronomy clicker question: Milky Way shape (The "High-Maintenance Camper's Dilemma")

Astronomy 10, Summer Session 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q11.2

Students were asked the following clicker question (Classroom Performance System, einstruction.com) near the beginning of their learning cycle:

[0.3 points.] How do we know that the Milky Way is shaped like a flat disk?
(A) Most other galaxies are shaped like flat disks, therefore the Milky Way must be shaped like a flat disk.
(B) It is possible to visually trace out its flat disk shape if the night sky is dark enough.
(C) Distant stars appear to be dimmer than nearby stars.
(D) (None of the above choices (A)-(C), as the Milky Way is not shaped like a flat disk.)

Correct answer: (B)

This question is asked after an in-class activity where students plot out the most distant stars visible in the Milky Way, and then find that this only represents a very tiny portion of our entire galaxy. This problem posed as the "camping dilemma," where if you had forgotten to bring a mirror with you on a camping trip, then how do you know what you look like? Similarly, with interstellar dust and gas obscuring the majority of the Milky Way, then how do we know the overall shape (and size) of our own galaxy?

Response (A) is analogous to a high-maintenance person looking at the other campers, and asking, "Do I look as bad as they do?" This inference, while plausible, is not necessarily valid, as other galaxies come in various sizes and forms (as would, presumably, the unkempt appearance of other campers). Response (C) is a result of the inverse square law, compounded by interstellar dust and gas, and would be true regardless of the shape and structure of the Milky Way.

Prompt students to recall that under ideal conditions, the Milky Way is seen as a dense band of dim stars across the sky. This is the primary evidence that our galaxy has a thin disk shape, as many stars can be seen in the plane of this disk, while only a sparse distribution of stars are seen in directions above and below the plane of the disk. (While the overall size of the Milky Way cannot be determined solely from this observation, the lack of interstellar dust and gas above and below the plane of the disk does allow for clear views of globular clusters in the halo of the Milky Way, which are used to determine the overall size of the Milky Way.)

Student responses
Section 8027
(A) : 2 students
(B) : 5 students
(C) : 6 students
(D) : 0 students